Ethanol is known to alter the biochemical and biophysical properties of cellular membranes. In this investigation, study has been made on the effects of ethanol on brain opiate (enkephalin) receptors and membrane fluidity. Ethanol, added to rat caudate membranes inhibited specific binding of enkephalin. The binding constant in the absence of ethanol was 1.5 nM, addition of ethanol (400 mM) gave a non-linear double reciprocal Lineweaver-Burk) plot with two binding constants, KD1 = 1.1 and KD2 = 0.55nM. Some of these changes have been associated with a change in the physical properties of membranes in the presence of ethanol. The change in membrane fluidity was measured in the presence of ethanol by the method of fluorescence polarization with 1,6-diphenyl 1, 3, 5-hexatriene (DPH) as a probe. The fluorescence polarization of DPH incorporated into the synaptosomes decreases with increasing concentration of ethanol (0-500 mM). The calculated rotational rates are higher in the presence of ethanol indicating the increase in membrane fluidity caused by ethanol. These results suggest that changes in membrane physical properties associated with ethanol produces altered brain receptor function.